I am dark blonde and have quite fragile hair that is just starting to thicken up and become healthy - I would like to go brunette... is this wise? and if so what is the best way of doing this?? thanks for any replies

If I was in your situation I would try semi-permanent color first.  That way, you can have a lovely brunette color without dealing a ton of damage to the hair.  Also, if you don't like the  way brunette looks on you...it can fade out.  Henna is another good option.